There are known thin film transistors (TFTs), which use a thin film semiconductor that is formed on a substrate. While TFTs are used for various integrated circuits, they attract much attention particularly as switching elements provided for each pixel of an electro-optical device, particularly an active matrix type liquid crystal display device and as driver elements formed in its peripheral circuit portion.
Although an amorphous silicon film is conveniently used for the TFTs, it has a problem of low-grade electrical characteristics. The characteristics of the TFT can be improved by using a crystalline silicon thin film. “Crystalline silicon” includes polycrystalline silicon, polysilicon, microcrystalline silicon, or the like. A crystalline silicon can be obtained by forming an amorphous silicon film and then crystallizing it by heating.
However, since the crystallization by heating takes more than 20 hours at a heating temperature higher than 600° C., it is difficult to use a glass substrate. For example, Corning 7059 glass, which is used for active liquid crystal display devices, has a glass strain point of 593° C. Therefore, as the size of a substrate becomes larger, heating at more than 600° C. will be problematic. That is, if a commonly used Corning 7059 glass substrate is subjected to a heat treatment at 600° C. or higher and for 20 hours or longer, it will exhibit remarkable contraction and bending.
To solve the above problem, it is necessary to perform a heat treatment at a temperature as low as possible. On the other hand, to increase the productivity, it is required that the time for the heat treatment step should be as short as possible.
Further, when an amorphous silicon film is crystallized by heating, the entire silicon film is crystallized; that is, it is impossible to effect partial crystallization nor control the crystallinity of a particular region.
To solve this problem, Japanese Unexamined Patent Publication Nos. 2-140915 and 2-260524 disclose techniques of effecting selective crystallization by artificially forming a portion or region where crystalline nuclei are to be generated in an amorphous silicon film and then subjecting the film to a heat treatment. These techniques are intended to form crystalline nuclei at a prescribed portion in an amorphous silicon film.
For example, the publication No. 2-140915 discloses a technique in which an aluminum layer is formed on an amorphous silicon film, crystalline nuclei are generated in the portion where amorphous silicon and aluminum are contacted with each other, and crystal growth is caused to proceed from the crystalline nuclei by a heat treatment. The 2-260524 publication discloses a technique in which tin (Sn) is added to an amorphous silicon film by ion implantation and crystalline nuclei are generated in a tin-ion-added region.
However, since Al and Sn are substitutional metal elements, they form an alloy with silicon and do not diffuse into a silicon film. Crystallization proceeds such that crystalline nuclei are generated in a portion where an alloy with silicon is formed and crystal growth is started from that portion. That is, the cases of using Al or Sn is characterized in that crystal growth starts from a portion where Al or Sn is introduced, i.e., from an alloy layer of that element and silicon. In general, crystallization is a two-step process consisting of generation of initial nuclei and crystal growth from that nuclei. Although Al and Sn, which are substitutional metal element with respect to silicon, are effective in generating initial nuclei, they are not effective in crystal growth that should follow.
Therefore, even if Al or Sn is used, the crystallization temperature cannot be lowered nor can the crystallization time be shortened from the case of crystallizing an amorphous silicon film simply by heating it. That is, the use of Al or Si has no advantage over the conventional process of crystallizing an amorphous silicon film simply by heating it.
In accordance with the investigation by the inventors of the present invention, crystallization can be performed for about 4 hours at 550° C. by employing a process in which a very small amount of an interstitial element with respect to silicon, such as nickel or palladium, is deposited on the surface of an amorphous silicon film and then heating is effected. This process facilitates not only the initial nucleus generating step but also the subsequent crystal growth, and can therefore greatly lower the heating temperature and shorten the heating time compared with the conventional case of using only heating.
A small amount of the above mentioned element (catalyst element for accelerating crystallization) may be introduced by plasma treatment, evaporation or ion implantation. The plasma treatment is a method in which in a parallel-plate type or positive-column-type plasma CVD apparatus, a catalyst element is added to an amorphous silicon film by using, as an electrode, a material containing the catalyst element and generating a plasma in an atmosphere of nitrogen, hydrogen, or the like.
Examples of the above metal element for accelerating crystallization are interstitial elements of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Ag and Au. These interstitial elements are diffused into a silicon film in a heat treatment process. Then, crystallization of silicon proceeds as the interstitial element diffuses. That is, the interstitial metal accelerates crystallization of an amorphous silicon film by its catalytic effect at every point which it reaches.
Therefore, the interstitial elements can cause crystallization to proceed in the manner different than in the case where crystallization gradually proceeds from crystalline nuclei. For example, where one of the above metal elements is introduced into an amorphous silicon film at a particular point and then a heat treatment is performed, crystallization proceeds parallel with the film surface from the portion where the metal element was introduced over more than several tens of micrometers. Where a metal element is introduced into an amorphous silicon film over its entire area, the entire film can be crystallized uniformly. In this case, although the entire film may result in a polycrystalline or microcrystalline structure, no clear grain boundary exists at a particular location. Therefore, it is possible to form devices having uniform characteristics in an arbitrary portion of the film.
Since the interstitial elements diffuse into a silicon film quickly, it is important to properly determine their introduction quantity (addition quantity). If the introduction quantity is too small, good crystallinity is not obtained because of an insufficient effect of accelerating crystallization. Conversely, if the introduction quantity is too large, semiconductor characteristics of silicon are degraded.
Thus, there exists an optimum introduction quantity with respect to an amorphous silicon film for each of the above-mentioned, metal elements. For example, where Ni is selected as a metal element for accelerating crystallization, the effect of accelerating crystallization is obtained if its concentration in a crystallized silicon film is more than 1×1015 cm−3, and that the semiconductor characteristics are not degraded if the concentration is less than 5×1019 cm−3. The concentration as mentioned above is defined as the minimum of values obtained by SIMS (secondary ion mass spectrometry). The above-mentioned metal elements other than Ni also exhibit their effect properly in respective concentration ranges similar to that of Ni.
So that the concentration of an element, such as Ni, for accelerating crystallization in a crystallized silicon film should fall within an optimum range, its quantity needs to be controlled when it is introduced into an amorphous silicon film.
Also, as to the case of using nickel as a catalyst element, the following facts have been found by a study in which amorphous silicon films were deposited, crystalline silicon films were then formed by adding Ni by a plasma treatment with detailed investigation of the crystallization process.
(1) When nickel is introduced into an amorphous silicon film by plasma processing, nickel has already been intruded into a considerable depth of the amorphous silicon film before a heat treatment is performed.
(2) Initial crystalline nuclei are generated at the surface through which nickel was introduced.
(3) Crystallization occurs even with a nickel coating deposited on an amorphous silicon film by evaporation in the same manner as in the case of using plasma processing.
It is concluded from the above facts that not all of nickel introduced by plasma processing functions effectively. That is, even if a large amount of nickel is introduced, part of it does not functions sufficiently. This leads to a conclusion that points (or a surface) where nickel and silicon are contacted with each other function in low-temperature crystallization. Therefore, it is necessary that nickel be dispersed in the form of as small particles as possible, preferably in the form of atoms. That is, it is concluded that nickel should be so introduced as to be dispersed in the form of atoms at as low a concentration as possible that allows low-temperature crystallization in a portion close to the surface of an amorphous silicon film.
Evaporation is a candidate of introducing a very small amount of nickel into only a portion close to the surface of an amorphous silicon film, in other words, introducing a very small amount of catalyst element so that crystallization is accelerated only in a portion close to the surface of an amorphous silicon film. However, evaporation has a problem of low controllability; that is, it is difficult to strictly control the introduction quantity of a catalyst element.
Further, the introduction quantity of a catalyst element needs to be as small as possible, which causes a problem that it is difficult to obtain a sufficient crystallization.